Semiconductor laser

ABSTRACT

A semiconductor laser includes a semiconductor substrate of a first conductivity type and having a front surface; a first semiconductor layer disposed on the front surface of the semiconductor substrate and having a refractive index that increases with distance from the semiconductor substrate; an active layer disposed on the first semiconductor layer; and a second semiconductor layer disposed on the active layer, having a refractive index that decreases with distance from the active layer, and having a ridge. In this laser, the refractive index distribution between the ridge and the substrate is asymmetrical about the active layer so that the center of the light intensity distribution shifts from the active layer toward the substrate, in the direction perpendicular to the front surface of the substrate. Therefore, propagated light is hardly affected by the refractive index distribution in the width direction of the laser, which is caused by the presence of the ridge, whereby occurrence of a higher mode is suppressed.

FIELD OF THE INVENTION

The present invention relates to semiconductor lasers and, more particularly, to a semiconductor laser used as a light source for information processing or optical communication.

BACKGROUND OF THE INVENTION

FIG. 9(a) is a cross-sectional view of a ridge type semiconductor laser disclosed by J. Hashimoto et.al. in IEEE Journal of Quantum Electron, Vol.33, pp.66-77, 1997. This laser comprises a p side electrode 1, an insulating film 2, a p type GaAs contact layer 3, a p type GaInP upper cladding layer 4 having a stripe-shaped ridge extending in the resonator length direction, a ridge side undoped GaInAsP second guide layer 5, a ridge side undoped GaAs first guide layer 6, an active layer 7, a substrate side undoped GaAs first guide layer 8 having the same composition ratio and thickness as those of the ridge side first guide layer 6, a substrate side undoped GaInAsP second guide layer 9 having the same composition ratio and thickness as those of the ridge side second guide layer 5, an n type GaInP lower cladding layer 10 having the same composition ratio as that of the upper cladding layer 4 and the same thickness as that of the ridge of the cladding layer 4, an n type GaAs buffer layer 11, an n type GaAs substrate 12, and an n side electrode 13. FIG. 9(b) is a graph showing the refractive index profile of the semiconductor laser in the direction perpendicular to the surface of the substrate 12. In FIG. 9(a), “z” shows the resonator length direction, “x” shows the direction perpendicular to the surface of the substrate 12 (hereinafter referred to as “thickness direction”), and “y” shows the direction perpendicular to both of the resonator length direction z and the thickness direction x (hereinafter referred to as “width direction”).

A description is given of the operation of the semiconductor laser. Holes and electrons are injected through the upper cladding layer 4 and the lower cladding layer 10 into the active layer 7, respectively, and recombine to generate light. The light so generated is propagated along the resonator length direction z while being influenced by the refractive indices in the thickness direction x and the width direction y, and it is amplified while being reflected between the facets of the laser, resulting in laser oscillation.

In this prior art semiconductor laser, the refractive index distribution in the thickness direction x is symmetrical about the active layer 7, until reaching the upper cladding layer 4 and the lower cladding layer 10 which are disposed on and beneath the active layer 7, respectively. That is, as shown in FIG. 9(b), the ridge side first guide layer 6, the ridge side second guide layer 5, and the upper cladding layer 4 have the same refractive indices and the same thicknesses as those of the substrate side first guide layer 8, the substrate side second guide layer 9, and the lower cladding layer 10, respectively.

As described above, in the prior art ridge type semiconductor laser, since the refractive index distribution in the thickness direction x is symmetrical about the active layer 7, the propagated light is distributed almost symmetrically about the active layer 7 in the thickness direction x. However, a refractive index difference is present in the width direction y because of the ridge of the upper cladding layer 4 when the propagated light is distributed almost symmetrically about the active layer 7 as described above, the influence of the refractive index in the width direction y on the propagated light at the ridge becomes significant, whereby a higher mode of oscillation is permitted. As a result, kinks occur due to mode competition during low-power output operation, and the output power cannot be increased in practical use.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a ridge type semiconductor laser that can shift the level at which a higher mode occurs, toward the higher power level.

Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.

According to a first aspect of the present invention, there is provided a semiconductor laser including a semiconductor substrate of a first conductivity type and having a front surface; a first semiconductor layer disposed on the front surface of the semiconductor substrate and having a refractive index that increases with distance from the semiconductor substrate; an active layer disposed on the first semiconductor layer; and a second semiconductor layer disposed on the active layer, having a refractive index that decreases with distance from the active layer, and having a ridge; wherein the refractive index distribution between the ridge and the substrate is asymmetrical about the active layer so that the center of the light intensity distribution shifts from the active layer toward the substrate, in the direction perpendicular to the front surface of the substrate. Therefore, more light is distributed to the substrate side than the ridge side, and the influence of the refractive index distribution in the width direction on propagated light is reduced, which distribution occurs due to a difference in refractive indices between the ridge of the second semiconductor layer and portions of the layer outside the ridge, thereby avoiding occurrence of a higher mode that causes kinks. As a result, the light output level at which kinks occur is increased, providing a ridge type semiconductor laser capable of high-power output operation in the practical use.

According to a second aspect of the present invention, in the above-mentioned semiconductor laser, the first semiconductor layer comprises a lower cladding layer of the first conductivity type and having a refractive index, and a substrate side guide layer disposed on the lower cladding layer and having a thickness and a refractive index; the second semiconductor layer comprises a ridge side guide layer having a thickness and a refractive index, and an upper cladding layer of a second conductivity type, opposite the first conductivity type, disposed on the ridge side guide layer and having a refractive index; and at least one of the thickness and the refractive index of the substrate side guide layer is larger than that of the ridge side guide layer. Therefore, more light is distributed to the substrate side, and the light output level at which kinks occur is increased, resulting in a ridge type semiconductor laser capable of high-power output operation in the practical use.

According to a third aspect of the present invention, in the above-mentioned semiconductor laser, the refractive index of the lower cladding layer is larger than that of the upper cladding layer. Therefore, more light is distributed to the substrate side, and the light output level at which kinks occur is increased, resulting in a ridge type semiconductor laser capable of high-power output operation in the practical use. In addition, the expansion of the far field pattern is reduced, whereby the aspect ratio of light is reduced.

According to a fourth aspect of the present invention, in the above-mentioned semiconductor laser, the first semiconductor layer includes a lower cladding layer of the first conductivity type and has a thickness and a refractive index; the second semiconductor layer includes an upper cladding layer of a second conductivity type, opposite the first conductivity type, and has a thickness and a refractive index; and at least one of the thickness and the refractive index of the lower cladding layer is larger than that of the upper cladding layer. Therefore, more light is distributed to the substrate side, and the light output level at which kinks occur is increased, resulting in a ridge type semiconductor laser capable of high-power output operation in the practical use. Especially when the refractive index of the lower cladding layer is increased, the expansion of the far field pattern is reduced, whereby the aspect ratio of light is reduced.

According to a fifth aspect of the present invention, in the above-mentioned semiconductor laser, the refractive index of the lower cladding layer continuously increases toward the active layer; and the refractive index of the upper cladding layer continuously decreases with distance from the active layer. Therefore, more light is distributed to the substrate side, and the light output level at which kinks occur is increased, resulting in a ridge type semiconductor laser capable of high-power output operation in the practical use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are diagrams for explaining a semiconductor laser according to a first embodiment of the present invention.

FIGS. 2(a) and 2(b) are diagrams for explaining a semiconductor laser according to a second embodiment of the present invention.

FIG. 3 is a diagram showing simulation results of the semiconductor laser according to the second embodiment.

FIGS. 4(a) and 4(b) are diagrams for explaining a semiconductor laser according to a third embodiment of the present invention.

FIGS. 5(a) and 5(b) are diagrams for explaining a semiconductor laser according to a fourth embodiment of the present invention.

FIGS. 6(a) and 6(b) are diagrams for explaining a semiconductor laser according to a fifth embodiment of the present invention.

FIG. 7 is a diagram showing simulation results of the semiconductor laser according to the fifth embodiment.

FIGS. 8(a) and 8(b) are diagrams for explaining a semiconductor laser according to a sixth embodiment of the present invention.

FIGS. 9(a) and 9(b) are diagrams for explaining a semiconductor laser according to the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

FIG. 1(a) is a cross-sectional view illustrating a semiconductor laser according to a first embodiment of the present invention, and FIG. 1(b) is a graph showing the refractive index profile of the laser in the thickness direction x. This laser includes an n type GaAs substrate 12. An n type AlGaAs lower cladding layer 10 a (first semiconductor layer) is disposed on the substrate 12. An undoped InGaAs active layer 7 a is disposed on the lower cladding layer 10 a. A p type AlGaAs upper cladding layer 4 a (second semiconductor layer) is disposed on the active layer 7 a. The upper cladding layer 4 a has a stripe-shaped ridge extending in the resonator length direction z, at its surface. A p type GaAs contact layer 3 is disposed on the ridge of the upper cladding layer 4 a. The surface of the upper cladding layer 4 a including the ridge is covered with an insulating film 2, such as a silicon oxide film, except the top of the contact layer 3. A p side electrode 1 is disposed on the insulating film 2 and on the contact layer 3. An n side electrode 13 is disposed on the rear surface of the substrate 12.

The thickness of the substrate side lower cladding layer 10 a is larger than the thickness of the ridge side upper cladding layer 4 a, and the refractive index of the lower cladding layer 10 a at a portion contacting the substrate is larger than the refractive index of the upper cladding layer 4 a at the top of the ridge. In this first embodiment, the difference in refractive indices between the lower cladding layer 10 a and the upper cladding layer 4 a is obtained by setting the Al composition ratio of the lower cladding layer 10 a smaller than the Al composition ratio of the upper cladding layer 4 a. In each of the cladding layers, the refractive index gradually decreases with distance from the active layer 7 a. Thereby, the refractive index distribution between the upper cladding layer 4 a and the lower cladding layer 10 a in the thickness direction x is asymmetrical about the active layer 7 a.

A description is given of the method of fabricating the semiconductor laser. Initially, the lower cladding layer 10 a, the active layer 7 a, the upper cladding layer 4 a, and the contact layer 3 are successively grown on the surface of the substrate 12. The lower cladding layer 10 a is grown so that the Al composition ratio continuously decreases, and the upper cladding layer 4 a is grown so that the Al composition ratio continuously increases.

Next, a stripe-shaped mask comprising an insulating film (not shown) is formed on the contact layer 3 and, using this mask, the contact layer 3 and an upper portion of the upper cladding layer 4 a are selectively etched to form a ridge. After removal of the mask, the insulating film 2 is formed over the surface of the structure, and a portion of the insulating film 2 at the top of the contact layer 3 is removed by etching. Further, the p side electrode 1 is formed on the insulating film 2, contacting the contact layer 3, and the n side electrode 13 is formed on the rear surface of the substrate 12. Finally, resonator facets are formed by cleaving, thereby completing the semiconductor laser shown in FIG. 1.

A description is given of the operation of the laser. Holes and electrons are injected into the active layer 7 a through the ridge of the upper cladding layer 4 a and the lower cladding layer 10 a, respectively, and recombine to generate light. The light so generated is transmitted in the resonator length direction z while being influenced by the refractive indices in the thickness direction x and the width direction y, and it is amplified while being reflected between the facets, resulting in laser oscillation.

In the semiconductor laser according to this first embodiment, as described above, the thickness and refractive index of the lower cladding layer 10 a on the substrate side are larger than those of the upper cladding layer 4 a on the ridge side so that the refractive index distribution in the thickness direction x becomes asymmetrical about the active layer 7 a. Since the refractive index distribution in the thickness direction x is so set, more light is distributed to the substrate side than the ridge side when viewed from the active layer 7 a. Therefore, the influence of the refractive index distribution in the width direction y on the propagated light is reduced, which distribution occurs due to a difference in refractive indices between the ridge of the upper cladding layer 4 a and portions of the layer 4 a outside the ridge, thereby avoiding occurrence of a higher mode that causes kinks. As a result, practical high-power operation is realized. In addition, only the fundamental mode is permitted even when the ridge width is increased.

In this first embodiment of the invention, the thickness and refractive index of the lower cladding layer 10 a are set larger than those of the upper cladding layer 4 a to make the refractive index distribution between the lower cladding layer 10 a and the upper cladding layer 4 a asymmetrical about the active layer 7 a. However, the present invention is not restricted thereto, and any structure may be adopted as long as the refractive index distribution becomes asymmetrical so that the center (peak) of the light intensity distribution shifts from the active layer 7 a toward the substrate 12. For example, one of the thickness and the refractive index of the lower cladding layer 10 a may be set larger than that of the upper cladding layer 4 a to obtain the asymmetric refractive index distribution.

Furthermore, although in this first embodiment the refractive indices of the upper and lower cladding layers 4 a and 10 a continuously increase toward the active layer 7 a, the refractive indices of the respective cladding layers may be fixed in the thickness direction x as long as the thickness and refractive index of the lower cladding layer are larger than those of the upper cladding layer.

Embodiment 2

FIG. 2(a) is a cross-sectional view illustrating a semiconductor laser according to a second embodiment of the invention, and FIG. 2(b) is a graph showing the refractive index profile of the laser in the thickness direction x. The semiconductor laser according to this second embodiment is different from the semiconductor laser according to the first embodiment in the following points. While in the first embodiment the lower cladding layer 10 a (first semiconductor layer) comprising a single semiconductor layer is disposed between the active layer 7 a and the substrate 12, in this second embodiment a first composite semiconductor layer is disposed between the active layer 7 a and the substrate 12, which composite layer comprises an n type AlGaAs lower cladding layer 10 b, a substrate side second guide layer 9 a comprising undoped AlGaAs and having a thickness g_(L2), and a substrate side first guide layer 8 a comprising undoped-GaAs and having a thickness g_(L1). Further, in place of the upper cladding layer 4 a (second semiconductor layer) on the active layer 7 a in the first embodiment, a second composite semiconductor layer is disposed on the active layer 7 a in this second embodiment, which composite layer comprises a ridge side first guide layer 6 a comprising undoped GaAs and having a thickness g_(U1), a ridge side second guide layer 5 a comprising undoped AlGaAs and having a thickness g_(U2), and a p type AlGaAs upper cladding layer 4 b having a ridge at its surface. The thickness g_(L1) of the substrate side first guide layer 8 a is larger than the thickness g_(U1) of the ridge side first guide layer 6 a, and the thickness g_(L2) of the substrate side second guide layer 9 a is larger than the thickness g_(U2) of the ridge side second guide layer 5 a. The lower cladding layer 10 b, the substrate side first guide layer 8 a, the substrate side second guide layer 9 a, the ridge side first guide layer 6 a, the ridge side second guide layer 5 a, and the upper cladding layer 4 b respectively have fixed refractive indices in the thickness direction x. The Al composition ratio of the substrate side second guide layer 9 a is smaller than the Al composition ratio of the lower cladding layer 10 b, and the Al composition ratio of the ridge side second guide layer 5 a is larger than the Al composition of the upper cladding layer 4 b. As a result, in the first composite semiconductor layer, the refractive index increases stepwise with distance from the substrate 12 and, in the second composite semiconductor layer, the refractive index decreases stepwise with distance from the active layer 7 a. In FIGS. 2(a) and 2(b), the same reference numerals as those in FIGS. 1(a) and 1(b) designate the same or corresponding parts. In FIG. 2(a), W denotes the ridge width, and t denotes the thickness of a portion of the upper cladding layer at each side of the ridge, i.e., a portion outside the ridge.

The fabrication method of the semiconductor laser according to this second embodiment is identical to the fabrication method already described for the first embodiment except that the lower cladding layer 10 b, the second guide layer 9 a, and the first guide layer 8 a are grown instead of the lower cladding layer 10 a, and the first guide layer 6 a, the second guide layer 5 a, and the upper cladding layer 4 b are grown instead of the upper cladding layer 4 a.

In the semiconductor laser according to this second embodiment, as shown in FIG. 2(b), the thickness g_(L1) of the substrate side first guide layer 8 a and the thickness g_(L2) of the substrate side second guide layer 9 a are larger than the thickness g_(U1) of the ridge side first guide layer 6 a and the thickness g_(U2) of the ridge side second guide layer 5 a, respectively, so that the refractive index distribution between the upper cladding layer 4 b and the lower cladding layer 10 b in the thickness direction x is asymmetrical about the active layer 7 a. As the result of the asymmetrical distribution of refractive index, the center of the light intensity distribution of the propagated light shifts from the active layer 7 a toward the substrate, so that more light is distributed to the substrate side. Therefore, the propagated light is hardly affected by the refractive index distribution in the width direction y due to the presence of the ridge, whereby occurrence of a higher mode is suppressed.

Hereinafter, the effects provided by increasing the thicknesses of the substrate side guide layers will be described on the basis of simulation results.

FIG. 3 is a graph showing the simulation results of the semiconductor laser according to the second embodiment. In FIG. 3, the ordinate shows the thickness t of the upper cladding layer 4 b outside the ridge, and the abscissa shows the ridge width W. Each of the continuous line and the dotted line shows the boundary between a region where only the fundamental mode (0 order) is permitted and a region where the primary mode (primary order) is also permitted, which boundary is calculated from the thickness t and the ridge width W using the equivalent refractive index method. The region where only the fundamental mode is permitted is under the boundary, and the region where the primary mode is also permitted is above the boundary. In this simulation, the Al composition ratio of the upper cladding layer 4 b is 0.3, the thickness of the ridge is t+1.4 μm, and the Al composition ratio of the ridge side second guide layer 5 a is 0.2. The active layer 7 a has a quantum well structure in which a 20 nm thick GaAs layer is put between two 8 nm thick InGaAs layers having an In composition ratio of 0.15. The Al composition ratio of the substrate side second guide layer 9 a is 0.2, and the Al composition ratio and the thickness of the lower cladding layer 10 a are 0.3 and t+1.4 μm, respectively. The simulation result shown by the dotted line is obtained when the ridge side first guide layer 6 a and the substrate side first guide layer 8 a have the same thickness of 0.02 μm, and the ridge side second guide layer Sa and the substrate side second guide layer 9 a have the same thickness of 0.04 μm, whereby the refractive index distribution is symmetrical about the active layer 7 a (symmetrical structure). The simulation result shown by the continuous line is obtained when the ridge side second guide layer 5 a is 0.04 μm thick, the ridge side first guide layer 6 a is 0.02 μm thick, the substrate side first guide layer 8 a is 0.04 μm thick, and the substrate side second guide layer 9 a is 0.06 μm thick, whereby the refractive index distribution is asymmetrical as shown in FIG. 2(b) (asymmetrical structure). It can be seen from FIG. 3 that the boundary between the fundamental mode and the primary mode shifts upward in the asymmetrical structure than in the symmetrical structure. For example, when t is 0.40 μm, in the symmetrical structure, the boundary between the region where only the fundamental mode is permitted and the region where the primary mode is also permitted is present at the ridge width W of about 3.65 μm. On the other hand, in the asymmetrical structure, when t is 0.40 μm, the boundary is present at the ridge width W of about 4.05 μm. In FIG. 3, in the region between the continuous line and the dotted line, kinks easily occur as both the fundamental mode and the primary mode are permitted in this region in the symmetrical structure. However, in the asymmetrical structure according to the second embodiment, since only the fundamental mode is permitted in this region, occurrence of kinks is suppressed.

While in this second embodiment the thicknesses of both of the first and second guide layers 8 a and 9 a on the substrate side are increased to shift the light intensity distribution toward the substrate side, the same effects as described above are achieved even when the thickness of one of the first and second guide layers 8 a and 9 a is increased.

Further, while in this second embodiment two guide layers are disposed at each of the upper and lower sides of the active layer 7 a, a single guide layer may be disposed at each side of the active layer 7 a. Even in this case, the same effects as described above are achieved by increasing the thickness of the substrate side guide layer.

Furthermore, even when three or more guide layers are disposed at each side of the active layer 7 a, the same effects as described above are achieved by increasing the thickness of at least one of the substrate side guide layers.

Moreover, while in this second embodiment the refractive indices of the respective guide layers disposed on and beneath the active layer 7 a are fixed in the thickness direction, these refractive indices may be continuously increased toward the active layer 7 a. Also in this case, the same effects as described above are achieved by increasing the thickness of the substrate side guide layer.

Embodiment 3

FIG. 4(a) is a cross-sectional view illustrating a semiconductor laser according to a third embodiment of the invention, and FIG. 4(b) is a graph showing the refractive index profile of the laser in the thickness direction x. The semiconductor laser according to this third embodiment is different from the semiconductor laser according to the first embodiment in the following points. While in the first embodiment the lower cladding layer 10 a (first semiconductor layer) comprising a single semiconductor layer is disposed between the active layer 7 a and the substrate 12, in this third embodiment a first composite semiconductor layer is disposed between the active layer 7 a and the substrate 12, which composite layer comprises an n type AlGaAs lower cladding layer 10 b, a substrate side second guide layer 9 b comprising undoped AlGaAs and having a refractive index n_(Lg2), and a substrate side first guide layer 8 b comprising undoped GaAs and having a refractive index n_(Lg1). Further, in place of the upper cladding layer 4 a (second semiconductor layer) on the active layer 7 a in the first embodiment, a second composite semiconductor layer is disposed on the active layer 7 a in this third embodiment, which composite layer comprises a ridge side first guide layer 6 b comprising undoped GaAs and having a refractive index n_(Ug1), a ridge side second guide layer 5 b comprising undoped AlGaAs and having a refractive index n_(Ug2), and a p type AlGaAs upper cladding layer 4 b having a ridge at its surface. The refractive index n_(Lg1) of the substrate side first guide layer 8 b is larger than the refractive index n_(Ug1) of the ridge side first guide layer 6 b, and the refractive index n_(Lg2) of the substrate side second guide layer 9 b is larger than the refractive index n_(Ug2) of the ridge side second guide layer 5 b. To set the refractive indices of these semiconductor layers as mentioned above, the Al composition ratios of the respective layers are controlled. The substrate side first guide layer 8 b, the substrate side second guide layer 9 b, the ridge side first guide layer 6 b, and the ridge side second guide layer 5 b respectively have fixed refractive indices in the thickness direction x. The Al composition ratio of the substrate side second guide layer 9 b is smaller than the Al composition ratio of the lower cladding layer 10 b, and the Al composition ratio of the ridge side second guide layer 5 b is larger than the Al composition ratio of the upper cladding layer 4 b. As a result, in the first composite semiconductor layer, the refractive index increases stepwise with distance from the substrate 12 and, in the second composite semiconductor layer, the refractive index decreases stepwise with distance from the active layer 7 a. In FIGS. 4(a) and 4(b), the same reference numerals as those shown in FIGS. 1(a) and 1(b) designate the same or corresponding parts.

The fabrication method of the semiconductor laser according to this third embodiment is identical to the fabrication method already described for the first embodiment except that the lower cladding layer 10 b, the substrate side the second guide layer 9 b, and the substrate side first guide layer 8 b are grown instead of the lower cladding layer 10 a, and the ridge side first guide layer 6 b, the ridge side second guide layer 5 b, and the upper cladding layer 4 b are grown instead of the upper cladding layer 4 a.

In the semiconductor laser according to this third embodiment, as shown in FIG. 4(b), the refractive index n_(Lg1) of the substrate side first guide layer 8 b and the refractive index n_(Lg2) of the substrate side second guide layer 9 b are larger than the refractive index n_(Ug1) of the ridge side first guide layer 6 b and the refractive index n_(Ug2) of the ridge side second guide layer 5 b, respectively, so that the refractive index distribution between the upper cladding layer 4 b and the lower cladding layer 10 b in the thickness direction x is asymmetrical about the active layer 7 a. As the result of the asymmetrical distribution of refractive index, the light intensity distribution of the propagated light shifts toward the substrate with respect to the active layer 7 a, and more light is distributed to the substrate side. Therefore, the light is hardly affected by the refractive index distribution in the width direction y which is caused by the presence of the ridge, whereby occurrence of a higher mode is suppressed.

While in this third embodiment the refractive indices of both of the substrate side first and second guide layers 8 b and 9 b are increased, the same effects as described above are achieved even when either of these refractive indices is increased.

Further, even when a single guide layer is disposed at each of the upper and lower sides of the active layer 7 a, the same effects as described above are achieved by increasing the refractive index of the substrate side guide layer.

Furthermore, even when three or more guide layers are disposed at each of the upper and lower sides of the active layer 7 a, the same effects as described above are achieved by increasing the refractive index of at least one of the substrate side (lower side) guide layers.

Moreover, while in this third embodiment the refractive indices of the respective guide layers disposed on and beneath the active layer 7 a are fixed in the thickness direction, these refractive indices may be continuously increased toward the active layer 7 a. Also in this case, the same effects as described above are achieved by increasing the refractive indices of the substrate side guide layers as a whole.

Embodiment 4

FIG. 5(a) is a cross-sectional view illustrating a semiconductor laser according to a fourth embodiment of the invention, and FIG. 5(b) is a graph showing the refractive index profile of the laser in the thickness direction x.

The semiconductor laser according to this fourth embodiment is different from the semiconductor laser according to the first embodiment in the following points. While in the first embodiment the lower cladding layer 1 a (first semiconductor layer) comprising a single semiconductor layer is disposed between the active layer 7 a and the substrate 12, in this fourth embodiment a first composite semiconductor layer is disposed between the active layer 7 a and the substrate 12, which composite layer comprises an n type AlGaAs lower cladding layer 10 b, a substrate side second guide layer 9 c comprising undoped AlGaAs and having a thickness g_(L2) and a refractive index n_(Lg2), and a substrate side first guide layer 8 c comprising undoped GaAs and having a thickness g_(L1) and a refractive index n_(Lg1). Further, in place of the upper cladding layer 4 a (second semiconductor layer) on the active layer 7 a in the first embodiment, a second composite semiconductor layer is disposed on the active layer 7 a in this third embodiment, which composite layer comprises a ridge side first guide layer 6 c comprising undoped GaAs and having a thickness g_(U1) and a refractive index n_(Ug1), a ridge side second guide layer 5 c comprising undoped AlGaAs and having a thickness g_(U2) and a refractive index n_(Ug2), and a p type AlGaAs upper cladding layer 4 b having a ridge at its surface. The thickness g_(Lg1) of the substrate side first guide layer 8 c is larger than the thickness g_(U2) of the ridge side first guide layer 6 c, and the thickness g_(L2) of the substrate side second guide layer 9 c is larger than the thickness g_(U2) of the ridge side second guide layer 5 c. The refractive index n_(Lg1) of the substrate side first guide layer 8 c is larger than the refractive index n_(Ug1) of the ridge side first guide layer 6 c, and the refractive index n_(Lg2) of the substrate side second guide layer 9 c is larger than the refractive index n_(Ug2) of the ridge side second guide layer 5 c. The substrate side first guide layer 8 c, the substrate side second guide layer 9 c, the ridge side first guide layer 6 c, and the ridge side second guide layer 5 c respectively have fixed refractive indices in the thickness direction x. The Al composition ratio of the substrate side second guide layer 9 c is smaller than the Al composition ratio of the lower cladding layer 10 b, and the Al composition ratio of the ridge side second guide layer 5 c is larger than the Al composition ratio of the upper cladding layer 4 b. As a result, in the first composite semiconductor layer, the refractive index increases stepwise with distance from the substrate 12 and, in the second composite semiconductor layer, the refractive index decreases stepwise with distance from the active layer 7 a. In FIGS. 5(a) and 5(b), the same reference numerals as those shown in FIGS. 1(a) and 1(b) designate the same or corresponding parts.

The fabrication method of the semiconductor laser according to this fourth embodiment is identical to the fabrication method already described for the first embodiment except that the lower cladding layer 10 b, the substrate side the second guide layer 9 c, and the substrate side first guide layer 8 c are successively grown instead of the lower cladding layer 10 a, and the ridge side first guide layer 6 c, the ridge side second guide layer 5 c, and the upper cladding layer 4 b are grown instead of the upper cladding layer 4 a.

In this semiconductor laser, as shown in FIG. 5(b), the thickness g_(L1) of the substrate side first guide layer 8 c and the thickness g_(L2) of the substrate side second guide layer 9 c are larger than the thickness g_(U1) of the ridge side first guide layer 6 c and the thickness g_(U2) of the ridge side second guide layer 5 c, respectively, and the refractive index n_(Lg1) of the substrate side first guide layer 8 c and the refractive index n_(Lg2) of the substrate side second guide layer 9 c are larger than the refractive index n_(Ug1) of the ridge side first guide layer 6 c and the refractive index n_(Ug2) of the ridge side second guide layer 5 c, respectively, whereby the refractive index distribution between the upper cladding layer 4 b and the lower cladding layer 10 b in the thickness direction x is asymmetrical about the active layer 7 a. Thereby, the center of the light intensity distribution of propagated light shifts from the active layer 7 a toward the substrate 12, so that more light is distributed to the substrate side than the ridge side. As a result, the propagated light is hardly affected by the refractive index distribution in the width direction y due to the presence of the ridge, and the same effects as provided by the first embodiment are achieved.

In this fourth embodiment of the invention, the thicknesses and refractive indices of the substrate side first and second guide layers 8 c and 9 c are set larger than those of the ridge side first and second guide layers 6 c and 5 c, respectively. However, the thickness and refractive index of either of the substrate side first and second guide layers 8 c and 9 c may be larger than those of the ridge side guide layer, with the same effects as described above.

Further, even when a single guide layer is disposed at each of the upper and lower sides of the active layer 7 a, the same effects as described above are obtained by setting the thickness and refractive index of the substrate side guide layer larger than those of the ridge side guide layer.

Furthermore, even when three or more guide layers are disposed at each of the upper and lower sides of the active layer 7, the same effects as mentioned above are obtained by setting the thickness and refractive index of at least one of the guide layers on the substrate side (lower side) larger than those of the ridge side (upper side) guide layer.

Moreover, although in this fourth embodiment the refractive indices of the respective guide layers disposed on and beneath the active layer 7 a are fixed in the thickness direction x, these refractive indices may be continuously increased toward the active layer 7 a. Also in this case, the same effects as described above are obtained when the refractive indices and thicknesses of the substrate side guide layers are larger than those of the ridge side guide layers.

Embodiment 5

FIG. 6(a) is a cross-sectional view of a semiconductor laser according to a fifth embodiment of the present invention, and FIG. 6(b) is a graph showing the refractive index profile of the laser in the thickness direction x.

The semiconductor laser according to this fifth embodiment is different from the semiconductor laser according to the first embodiment in the following points. While in the first embodiment the lower cladding layer 10 a (first semiconductor layer) comprising a single semiconductor layer is disposed between the active layer 7 a and the substrate 12, in this fifth embodiment a first composite semiconductor layer is disposed between the active layer 7 a and the substrate 12, which composite layer comprises an n type AlGaAs lower cladding layer 10 c having a refractive index n_(Lc), a substrate side second guide layer 9 d comprising undoped AlGaAs and having a thickness g_(L2) and a refractive index n_(Lg2), and a substrate side first guide layer 8 d comprising undoped GaAs and having a thickness g_(L1) and a refractive index n_(Lg1). Further, in place of the upper cladding layer 4 a (second semiconductor layer) on the active layer 7 a in the first embodiment, a second composite semiconductor layer is disposed on the active layer 7 a in this fifth embodiment, which composite layer comprises a ridge side first guide layer 6 d comprising undoped GaAs and having the same refractive index and thickness as those of the substrate side first guide layer 8 d, a ridge side second guide layer 5 d comprising undoped AlGaAs and having the same thickness and refractive index as those of the substrate side second guide layer 9 d, and a p type AlGaAs upper cladding layer 4 c having a refractive index n_(Uc) and a ridge at its surface. In this fifth embodiment, the Al composition ratios of the lower cladding layer 10 c and the upper cladding layer 4 c are different from each other so that the refractive index n_(Lc) of the lower cladding layer 10 c is larger than the refractive index n_(Uc) of the upper cladding layer 4 c. The lower cladding layer 10 c, the substrate side second guide layer 9 d, the substrate side first guide layer 8 d, the ridge side first guide layer 6 d, the ridge side second guide layer 5 d, and the upper cladding layer 4 c respectively have fixed refractive indices in the thickness direction x. The Al composition ratio of the substrate side second guide layer 9 d is smaller than the Al composition ratio of the lower cladding layer 10 c. As a result, in the first composite semiconductor layer, the refractive index increases stepwise with distance from the substrate 12 and, in the second composite semiconductor layer, the refractive index decreases stepwise with distance from the active layer 7 a. In FIGS. 6(a) and 6(b), the same reference numerals as those shown in FIGS. 1(a) and 1(b) designate the same or corresponding parts.

The fabrication method of the semiconductor laser according to this fifth embodiment is identical to the fabrication method already described for the first embodiment except that the lower cladding layer 10 c, the substrate side the second guide layer 9 d, and the substrate side first guide layer 8 d are successively grown instead of the lower cladding layer 10 a, and the ridge side first guide layer 6 d, the ridge side second guide layer 5 d, and the upper cladding layer 4 c are successively grown instead of the upper cladding layer 4 a.

In this semiconductor laser, since the refractive index of the lower cladding layer 10 c is larger than the refractive index of the upper cladding layer 4 c as shown in FIG. 6(c), the refractive index distribution between the upper cladding layer 4 c and the lower cladding layer 10 c in the thickness direction x is asymmetrical about the active layer 7 a. Thereby, the center of the light intensity distribution of the propagated light shifts from the active layer 7 a toward the substrate 12, so that more light is distributed to the substrate side than the ridge side. As a result, the propagated light is hardly affected by the refractive index distribution in the width direction y due to the presence of the ridge, and the same effects as provided by the first embodiment are achieved. Further, in this fifth embodiment, since more light is distributed to the lower cladding layer 10 c, the waveguide pattern of light itself is extended, so that the far field pattern (FFP) in the thickness direction x is narrowed. As a result, the aspect ratio, i.e., the ratio of the thickness direction FFP to the width direction FFP, is reduced.

Next, the effects provided by increasing the refractive index of the substrate side lower cladding layer 10 c will be described on the basis of simulation results.

FIG. 7 is a graph showing the simulation results of the semiconductor laser according to this fifth embodiment. In FIG. 7, the ordinate shows the thickness t of the upper cladding layer 4 c outside the ridge, and the abscissa shows the ridge width W. Each of the continuous line and the dotted line shows the boundary between a region where only the fundamental mode (0 order) is permitted and a region where the primary mode (primary order) is also permitted, which boundary is calculated from the thickness t and the ridge width W using the equivalent refractive index method. The region where only the fundamental mode is permitted is under the boundary, and the region where the primary mode is also permitted is above the boundary. In this simulation, the Al composition ratio of the upper cladding layer 4 c is 0.3, the thickness of the upper cladding layer 4 c at the ridge is t+1.4 μm, and the Al composition ratio of the ridge side second guide layer 5 a is 0.2. The active layer 7 a has a quantum well structure in which a 20 nm thick GaAs layer is put between two InGaAs layers each having an In composition ratio of 0.15 and a thickness of 8 nm. The Al composition ratio of the substrate side second guide layer 9 d is 0.2, the Al composition ratio of the lower cladding layer 10 c is 0.3 or 0.28, and the thickness of the lower cladding layer 10 c is t+1.4 μm. The simulation result shown by the dotted line is obtained when the refractive index distribution is symmetrical about the active layer 7 a, i.e., when the Al composition ratio of the upper cladding layer 4 c is equal to that of the lower cladding layer 10 c and, therefore, these cladding layers have the same refractive index (n_(Lc)=n_(Uc)) The simulation result shown by the continuous line is obtained when the refractive index distribution is asymmetrical about the active layer 7 a, i.e., when the Al composition ratio of the lower cladding layer 10 c is smaller than that of the upper cladding layer 4 c and, therefore, the refractive index of the lower cladding layer 10 c is larger than that of the upper cladding layer 4 c (n_(Lc)>n_(Uc)).

It can be seen from FIG. 7 that, when the refractive indices of the upper and lower cladding layers are asymmetrical, the boundary between the fundamental mode and the primary mode shifts upward than when the refractive indices of the upper and lower cladding layers are symmetrical. For example, when t=0.40 μm, the boundary is positioned at the ridge width W of about 3.5 μm in the symmetrical structure, while the boundary is positioned at the ridge width W of about 4.15 μm in the asymmetrical structure. Therefore, the same effects as provided by the second embodiment are achieved.

In this fifth embodiment, only the refractive index of the lower cladding layer 10 c is set larger than the refractive index of the upper cladding layer 4 c to make the refractive index distribution asymmetrical about the active layer 7 a. However, in addition to the asymmetrical refractive indices of the upper and lower cladding layers, the refractive indices and thicknesses of the substrate side first and second guide layers may be set larger than those of the ridge side upper and lower guide layers as described for the fourth embodiment of the invention.

Embodiment 6

FIG. 8(a) is a cross-sectional view of a semiconductor laser according to a sixth embodiment of the present invention, and FIG. 8(b) is a graph showing the refractive index profile of the laser in the thickness direction x.

The semiconductor laser according to this sixth embodiment is identical to the semiconductor laser according to the fifth embodiment except that n type GaAs current blocking layers 14 are disposed on the upper cladding layer 4 c, contacting both sides of the ridge of the cladding layer 4 c, thereby to realize a gain waveguide type buried ridge semiconductor laser. Further, insulating films 2 a are disposed on the current blocking layers 14 and on the contact layer 3 except a portion of the top of the contact layer 3, and a p side electrode 1 a is disposed contacting the contact layer 3 at the top. This semiconductor laser is fabricated as follows. That is, in the fabrication process of the semiconductor laser according to the fifth embodiment, after the ridge is formed by selective etching with an insulating film as a mask, the current blocking layers 14 are selectively grown using the mask.

The buried ridge type semiconductor laser according to this sixth embodiment provides the same effects as provided by the fifth embodiment of the invention.

In this sixth embodiment, emphasis has been placed on a buried ridge type semiconductor laser obtained by burying the ridge structure of the semiconductor laser according to the fifth embodiment with the current blocking layers 14. However, a buried ridge type semiconductor laser obtained by burying the ridge structure of the semiconductor laser according to any of the first to fourth embodiments with the current blocking layers 14 is also within the scope of the invention.

Furthermore, although in this sixth embodiment the current blocking layers 14 comprise n type GaAs, current blocking layers comprising other materials may be adopted with the same effects as described above. For example, n type AlGaAs current blocking layers having an Al composition ratio higher than that of the upper cladding layer may be adopted to provide a refractive index waveguide type semiconductor laser.

In the first to sixth embodiments of the invention, the refractive index distribution viewed from the active layer is made asymmetrical by changing the thickness or the refractive index of at least one of the lower cladding layer, the substrate side first and second guide layers, the ridge side first and second guide layers, and the upper cladding layer, which are constituents of the first semiconductor laser between the active layer and the substrate and the second semiconductor layer disposed on the active layer and having a ridge at its surface. However, in the present invention, the same effects as those provided by the first to sixth embodiments are achieved as long as the refractive indices of the first semiconductor layer and the second semiconductor layer are respectively fixed in the thickness direction or are gradually decreased with distance from the active layer, and the refractive index distribution in the thickness direction is asymmetrical about the active layer so that the center of the light intensity distribution shifts from the active layer toward the substrate.

While a refractive index waveguide type semiconductor laser is described in the first to fifth embodiment and a gain waveguide type semiconductor laser is described in the sixth embodiment, the same effects are achieved in the present invention regardless of the type of the semiconductor laser.

Furthermore, while in the first to sixth embodiments of the invention AlGaAs is employed as the material of the lower cladding layer, the substrate side first and second guide layers, the ridge side first and second guide layers, and the upper cladding layer, other materials may be employed with the same effects as provided by these embodiments.

Moreover, while in the first to sixth embodiments GaAs is employed as the material of the substrate, other materials may be employed with the same effects as provided by these embodiments. 

What is claimed is:
 1. A semiconductor laser including: a semiconductor substrate of a first conductivity type and having a top surface; a first semiconductor layer disposed on the top surface of said semiconductor substrate; an active layer disposed on said first semiconductor layer, wherein said first semiconductor layer has a refractive index that increases with distance, along a thickness direction of said active layer, from said semiconductor substrate to said active layer; and a second semiconductor layer disposed on said active layer and having a ridge with a top surface at a thickest part of said second semiconductor layer, the second semiconductor layer having a refractive index that decreases in the thickness direction with distance from said active layer to said top surface of said ridge, wherein the refractive index distribution from said top surface of said ridge to said top surface of said semiconductor substrate is asymmetrical with respect to said active layer so that light generated in said semiconductor laser has an intensity distribution between said top surface of said ridge and said top surface of said semiconductor substrate that has a center shifted from said active layer toward said semiconductor substrate, along the thickness direction.
 2. The semiconductor laser of claim 1 wherein: the refractive index of said first semiconductor layer monotonically increases with distance in the thickness direction from said top surface of said semiconductor substrate toward said active layer; and the refractive index of said second semiconductor layer monotonically decreases with distance in the thickness direction toward said top surface of said ridge from said active layer.
 3. The semiconductor laser of claim 1 comprising a current blocking layer disposed on said second semiconductor layer at opposite sides of and contacting said ridge, said current blocking layer having a refractive index smaller than the refractive index of said second semiconductor layer, thereby defining a refractive index waveguiding structure transverse to the thickness direction. 